It is known that stereophonic recordings are most commonly implemented using a multiplicity of microphones each placed in the near-field of one or more musical instruments, and when the recordings are reproduced by conventional stereophonic sound systems comprising two stereo speakers, the perceived sound stage is substantially limited to the physical space separating the stereo speakers. Several signal processing systems have been devised to enhance the perceived size, or sonic dimension, of the reproduced sound stage, each having specific performance limitations. Signal processing systems have been developed employing crosstalk cancellation, in which a time-delayed, filtered and attenuated left stereo signal is mixed with the right stereo signal, and a time-delayed, filtered and attenuated right stereo signal is mixed with the left stereo signal, thereby enabling, under ideal conditions, the cancellation of acoustic crosstalk occurring from the left stereo speaker to the right ear of a listener and from the right stereo speaker to the left ear of the listener. While the above crosstalk cancellation systems provide an enhancement of sonic dimension, th extent of the enhancement is restricted with stereophonic, non-binaural recordings and requires that the listener remains precisely equidistant between the stereo speakers for proper crosstalk cancellation. Other signal processing systems have been devised employing difference signal enhancement, in which a derived L-R stereo difference signal is filtered, attenuated and mixed with the left stereo signal, and a derived R-L stereo difference signal is filtered, attenuated and mixed with the right stereo signal, resulting in an increase in the relative level of indirect sound components. Such systems provide an enhancement of sonic dimension, however the dimensional enhancement is invariably gained at the expense of a corresponding degradation in the imaging and focus of center-mixed musical instruments recorded with substantially equal amplitude on the left and right channels.
Variations of such difference signal enhancement systems incorporate means to dynamically compress, in real time, changes in the ratio of amplitude of the derived stereo difference signal relative to the unmodified stereo signals, thereby dynamically compressing changes in the ratio of indirect-to-direct sound components. Another variation of such difference signal enhancement systems incorporates a means to detect frequency bands of such difference signals that are relatively low in amplitude and selectively amplify such difference signal frequency bands, thereby providing a more complex method for dynamically compressing changes in the ratio of indirect-to-direct sound components. Enhancement systems providing dynamic compression of such ratio, however, exacerbate the above described degradation in the imaging and focus of center-mixed instruments for reasons later explained in the present invention.
An analysis of the characteristics of signal components created in stereophonic recordings indicates that a difference signal enhancement system, in which a dynamic expansion, rather than a dynamic compression, of changes in the ratio of indirect-to-direct sound components yields substantially greater sonic dimension relative to the above described prior art without degradation of they imaging and focus of center-mixed instruments. The expansion of changes in such ratio represents a diametrically opposite approach to that of any difference signal enhancement system known to this inventor in which such ratio is dynamically modified, and forms the functional basis of the present invention.
Empirical tests conducted by this inventor confirm that when an instrument is monitored by a microphone in the near-field, providing a low ratio of indirect-to-direct sound components, is center-mixed and dominant in amplitude (which conditions typically occur with single lead instruments and vocals), the nature of psychoacoustics is such that the listener anticipates the perception of a near-field reproduction of such instrument in the form of a convergent image located between the stereo speakers. Such conditions are optimally satisfied by a low level of difference signal enhancement, since a high level of difference signal enhancement would alter the reproduced near-field characteristics to those resembling far-field characteristics and thereby degrade the desired perception of a center image. Conversely, when instruments are recorded with reverberation to emulate monitoring by a microphone in the far-field, providing a higher ratio of indirect-to-direct sound components, or when instruments are mixed asymmetrically between the left and right channels (which conditions typically occur with groups of non-dominant instruments and vocals), the listener anticipates the perception of a far-field reproduction of such instruments in the form of a divergent image expanding beyond the physical space separating the stereo speakers. Such conditions are optimally satisfied by a high level of difference signal enhancement providing a reinforcement of far-field reverberant information and a divergent, non-specific image.
It follows from the above that a real-time expansion of changes in the ratio of indirect-to-direct sound components optimizes both the sonic dimension and imaging qualities of the reproduced stereophonic sound. Therefore, a feature of the present invention is a means for detecting the continually changing value of amplitude ratio of a derived stereo difference signal relative to a derived stereo sum signal, which correlates to the ratio of indirect-to-direct sound components, and a means for dynamically expanding, in real-time, changes in such ratio in the left-front and right-front output signals.
A characteristic of all difference signal enhancement processors known to this inventor is an inherent functionality of increasing input-to-output gain corresponding to increasing values of amplitude ratio of a derived stereo difference signal relative to a derived stereo sum signal. Therefore, an additional feature of the present invention is a means for utilizing such ratio-value to control a reduction in amplitude of the front-left and front-right output signals.
A second characteristic of all difference signal enhancement processes known to this inventor is a functionality of decreasing relative amplitude of bass frequencies in the processed output signals corresponding to increasing values of amplitude ratio of a derived stereo difference signal relative to a derived stereo sum signal. Therefore, yet another feature of the present invention is a means for utilizing such ratio-value to control a boost in the relative amplitude of bass frequencies in the left-front and right-front output signals; and, for the same reason, still another feature of the present invention is a means for utilizing the ratio-value to control a boost in amplitude of a derived L+R sum signal applied to a low-pass filter that generates a subwoofer output signal.
A further feature of the present invention is a means for utilizing the ratio-value to control a boost in amplitude of the derived L-R stereo difference signal that generates a left-rear output signal; and, a means for utilizing the ratio-value to control a boost in amplitude of the derived R-L stereo difference signal that generates a right-rear output signal.
An enhancement system incorporating at least one of the above described features of the present invention would be equally effective utilized in the recording process to apply its associated signal processes to the recorded information, as it would be in the playback process to apply such processes to unmodified recorded stereophonic signals.